Living-body information measurement device

ABSTRACT

Provided is a living-body information measurement device including a first light emitting element and a second light emitting element each that emits different light in wavelength, a light receiving element that receives the light emitted from the first and second light emitting elements and outputs a received light signal corresponding to an amount of the received light, a separation unit that separates the received light signal into a first received light signal and a second received light signal, a filter that removes noise components of the first received light signal and the second received light signal, and a measuring unit that measures plural living-body information using the first received light signal before the noise components are removed by the filter, the first received light signal whose noise components are removed by the filter, and the second received light signal whose noise components are removed by the filter.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority under 35 USC 119 from Japanese Patent Application Nos. 2016-064454 filed Mar. 28, 2016, 2016-064455 filed Mar. 28, 2016, and 2016-064456 filed Mar. 28, 2016.

BACKGROUND Technical Field

The present disclosure relates to a living-body information measurement device.

SUMMARY

According to an aspect of the invention, there is provided a living-body information measurement device including:

a first light emitting element and a second light emitting element each that emits different light in wavelength;

a light receiving element that receives the light emitted from the first light emitting element and the second light emitting element and outputs a received light signal corresponding to an amount of the received light;

a separation unit that separates the received light signal into a first received light signal corresponding to the amount of received light emitted from the first light emitting element and a second received light signal corresponding to the amount of received light emitted from the second light emitting element;

a filter that removes noise components of the first received light signal and the second received light signal; and

a measuring unit that measures plural living-body information using the first received light signal before the noise components are removed by the filter, the first received light signal whose noise components are removed by the filter, and the second received light signal whose noise components are removed by the filter.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the present disclosure will be described in detail based on the following figures, wherein:

FIG. 1 is a schematic diagram illustrating a measurement example of blood flow information and an oxygen saturation in the blood;

FIG. 2 is a graph illustrating one example of a change in an amount of received light caused by reflected light from a living body;

FIG. 3 is a schematic diagram used to explain a Doppler shift which occurs when a blood vessel is irradiated with a laser beam;

FIG. 9 is a schematic diagram used to explain speckles which occur when a blood vessel is irradiated with a laser beam;

FIG. 5 is a graph illustrating one example of a spectrum distribution with respect to a change in an amount of received light;

FIG. 6 is a graph illustrating one example of a change in the blood flow;

FIG. 7 is a graph illustrating one example of a change in an absorbance of light in a living body;

FIG. 8 is a view illustrating the configuration of a living-body information measurement device;

FIG. 9 is a view illustrating one example of an arrangement of a light emitting element and a light receiving element;

FIG. 10 is a view illustrating another example of an arrangement of a light emitting element and a light receiving element;

FIG. 11 is a timing chart illustrating one example of an emission timing of a light emitting element emitting IR light and a light emitting element emitting red light, and a light receiving timing of a light receiving element; and

FIG. 12 is a graph illustrating one example of an output waveform with a change in a cut-off frequency of an LPF.

DETAILED DESCRIPTION

Hereinafter, exemplary embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. Throughout the drawings, the same elements, operations or functions are denoted by the same reference numerals or symbols and explanation thereof will not be repeated for the purpose of brevity.

First, referring to FIG. 1, a method of measuring a blood flow information and an oxygen saturation in the blood, as one example of living-body information on the blood among living-body information, will be described with reference to FIG. 1.

As illustrated in FIG. 1, when light is emitted from a light emitting element 1 to penetrate through the body of a patient (a living body 8) and is received in a light receiving element 3, blood flow information and an oxygen saturation in the blood are measured by using the intensity of light reflected by or transmitted through arteries 4, veins 5 and capillaries 6 spread throughout the living body 8, i.e., measured using the amount of reflected or transmitted light received in the light receiving element 3.

(Measurement of Blood Flow Information)

FIG. 2 is one example of a curve 80 that represents the amount of reflected light received by the light receiving element 3. In the graph of FIG. 2, the horizontal axis represents time and the vertical axis represents an output of the light receiving element 3, i.e., the amount of light received by the light receiving element 3.

As illustrated in FIG. 2, the amount of light received in the light receiving element 3 is changed with time. This phenomenon maybe attributed to three optical phenomenons appearing when the living body 8 including the blood vessels is irradiated with light.

The first optical phenomenon is a change in absorption of light due to a change in volume of blood existing in a blood vessel under measurement by pulsation. The blood contains blood cells such as red blood cells and moves through the blood vessels such as capillaries 6. Therefore, the number of blood cells moving through the blood vessels may be changed with the change in the volume of the blood, which may have an influence on the amount of light received in the light receiving element 3.

As the second optical phenomenon, an influence by a Doppler shift may be considered.

As illustrated in FIG. 3, for example, when a region including the capillaries 6 as one example of the blood vessels is irradiated with a coherent light 40 of a frequency ω₀ such as a laser beam from the light emitting element 1, a scattered light 42 scattered by the blood cells moving through the capillaries 6 causes a Doppler shift having a frequency difference Δω₀ determined by a moving speed of the blood cells. In the meantime, the scattered light 42 scattered by the tissues (stationary tissues) such as the skins which do not contain moving bodies such as blood cells maintains the same frequency ω₀ as the irradiated laser beam. Therefore, the frequency ω₀+Δω₀ of the laser beam scattered by the blood vessels such as the capillaries 6 interferes with the frequency ω₀ of the laser beam scattered by the stationary tissues. Due to such interference, a beat signal having the frequency difference Δω₀ is generated and observed in the light receiving element 3, and as a result, the amount of light received in the light receiving element 3 is changed with time. The frequency difference Δω₀ of the beat signal observed in the light receiving element 3 falls within a frequency range having the upper limit of about several tens kHz, although the frequency difference Δω₀ depends on the moving speed of the blood cells.

The third optical phenomenon may be an influence by speckles.

As illustrated in FIG. 4, when the blood cells 7 such as the red blood cells moving through a blood vessel in a direction indicated by an arrow 44 are irradiated with coherent light 40 such as a laser beam from the light emitting element 1, the laser beam striking on the blood cells 7 is scattered in different directions. The scattered beams have different phases and accordingly interfere with one another in a random manner. This results in a light intensity distribution having a random spotted patterns. The light intensity distribution pattern formed in this way is called a “speckle pattern.”

As described above, since the blood cells 7 move through the blood vessel, a state of light scattering in the blood cells 7 is changed and accordingly the speckle pattern is changed with time. As a result, the amount of light received in the light receiving element 3 is changed with time.

Next, one example of a method of obtaining information on a blood flow will be described. When the amount of received light of the light receiving element 3 changed with time is obtained as illustrated in FIG. 2, the data included in a range of unit time T₀ are extracted and then subjected to, for example, the fast Fourier transform (FFT), thereby obtaining a spectrum distribution for each frequency ω. FIG. 5 is a graph showing a curve 82 representing an example of the spectrum distribution for each frequency ω in the unit time T₀. In the graph 82 of FIG. 5, the horizontal axis represents a frequency ω and the vertical axis represents a spectrum intensity.

Here, the blood volume is proportional to a value obtained by normalizing the area of power spectrum, which is indicated by a hatched region 84 surrounded by the horizontal axis and the vertical axis of the curve 82, with the total light amount. In addition, since a blood velocity is proportional to a frequency average of the power spectrum represented by the curve 82, the blood velocity is proportional to a value obtained by dividing a value, which is obtained by integrating a product of the frequency ω and the power spectrum at the frequency ω with respect to the frequency ω, by the area of the hatched region 84.

In addition, since the blood flow is represented by a product of the blood volume and the blood velocity, the blood flow may be obtained from a calculation formula of the blood volume and the blood velocity. The blood flow, the blood velocity and the blood volume are one example of the blood flow information without being limited thereto.

FIG. 6 is a graph showing one example of a curve 86 representing an example of the calculated change in the blood flow per unit time T₀. In the graph of FIG. 6, the horizontal axis represents a time and the vertical axis represents a blood flow.

As illustrated in FIG. 6, while the blood flow varies with time, the trend of variation maybe classified into two types. For example, in FIG. 6, a variation range 90 of the blood flow in an interval T₂ is larger than a variation range 88 of the blood flow in an interval T₁. This may be because the change of the blood flow in the interval T₁ is mainly due to the motion of a pulse, whereas the change of the blood flow in the interval T₂ is due to, for example, the congestion, the effect by the autonomic neuron or the like.

(Measurement of Oxygen Saturation)

Next, measurement of an oxygen saturation in the blood will be described. The oxygen saturation in the blood is an indicator that indicates a degree of hemoglobin bonded to oxygen in blood. As the oxygen saturation is reduced in the blood, a symptom such as anemia or the like is apt to occur.

FIG. 7 is a conceptual graph illustrating the change in absorbance of light absorbed in, for example, the living body 8. As Illustrated in FIG. 7, amount of light absorbed in the living body 8 shows a tendency of variation with time.

In addition, referring to the contents of the variation of amount of light absorbed in the living body 8, it is known that the amount of light absorbed is mainly varied by the arteries 4 but may be negligible in other tissues including the veins 5 and the stationary tissues as compared to the arteries 4. This is because the arterial blood pumped from the heart moves through the blood vessels with a pulse wave and the arteries 4 expand/contract along the sectional direction of the arteries 4 with time, thereby causing a change in thickness of the arteries 4. In FIG. 7, the range indicated by an arrow 94 represents a variation of amount of light absorbed corresponding to the change in thickness of the arteries 4.

In FIG. 7, assuming that the amount of received light at time t_(a) is I_(a) and the amount of received light at time t_(b) is I_(b), a variation ΔA of amount of light absorbed due to the change in thickness of the arteries 4 is expressed by the following equation (1)

ΔA=ln(I _(b) /I _(a))   (1)

In the meantime, it is known that the hemoglobin bonded to the oxygen flowing through the arteries 4 (oxidized hemoglobin) is apt to absorb light of an infrared (IR) region having a wavelength of about 880 nm or so and the hemoglobin not bonded to the oxygen (reduced hemoglobin) is apt to absorb light of a red region having a wavelength of about 665 nm or so. Further, it is known that the oxygen saturation has a proportional relationship with a ratio of the variation ΔA of amount of light absorbed at different wavelengths.

Accordingly, in comparison with other combinations of wavelengths, by using the infrared light (IR light) and the red light, which are likely to produce a difference in amount of light absorbed between the oxidized hemoglobin and the reduced hemoglobin, to calculate a ratio of variation ΔA_(Red) of amount of light absorbed when the living body 8 is irradiated with the IR light to variation ΔA_(IR) of amount of light absorbed when the living body 8 is irradiated with the red light, the oxygen saturation S calculated according to the following equation (2). In the equation (2), k is a proportional constant.

S=k(ΔA _(Red) /ΔA _(IR))   (2)

That is, when the oxygen saturation in the blood is calculated, plural light emitting elements 1 emitting light having different wavelengths, specifically, a light emitting element 1 emitting IR light and a light emitting element 1 emitting red light, are caused to emit light in such a manner that their light-emission periods do not overlap with each other, although the light-emission periods may partially overlap with each other. Then, the reflected light or transmitted light by each light emitting element 1 is received in the light receiving element 3 and the oxygen saturation in the blood is measured by calculating the equations (1) and (2) or known equations obtained by modifying these equations (1) and (2) from the amount of received light at respective light receiving points.

As a known equation obtained by modifying the equation (1), the variation ΔA of amount of light absorbed may be expressed as the following equation (3) by deploying the equation (1).

ΔA=lnI _(b)−lnI _(a)   (3)

In addition, the equation (1) may be modified into the following equation (4).

ΔA=ln(I _(b) /I _(a))=ln(1+(I _(b) −I _(a))/I _(a))   (4)

Typically, since the relation of ln(I_(b)/I_(a))≈(I_(b)−I_(a))/I_(a) is established from the relation of (I_(b)−I_(a))<</I_(a), the equation (1) may be replaced with the following equation (5) as the variation ΔA of amount of light absorbed.

ΔA≈(I _(b) −I _(a))/I _(a)   (5)

Hereinafter, when the light emitting element 1 emitting IR light and the light emitting element 1 emitting red light are required to be distinguished from each other, the light emitting element 1 emitting IR light will be referred to as a “light emitting element LD1” and the light emitting element 1 emitting red light will be referred to as a “light emitting element LD2.” In addition, as one example, the light emitting element LD1 is assumed as the light emitting element 1 used for calculation of the blood flow and the light emitting elements LD1 and LD2 are assumed as light emitting elements 1 used for calculation of the oxygen saturation in the blood.

In addition, when the oxygen saturation in the blood is measured, since it is known that a frequency of measurement of the amount of received light is sufficient to fall within a range of from about 30 Hz to about 1,000 Hz, the emission frequency of the light emitting element LD2, which indicates the number of times of flickering per one second, is also sufficient to fall within a range of from about 30 Hz to about 1,000 Hz. Therefore, from the standpoints of power consumption of the light emitting element LD2, although the emission frequency of the light emitting element LD2 may be preferably set to be lower than the emission frequency of the light emitting element LD1, it may be set that the light emitting element LD1 and the light emitting element LD2 emit light alternately with the emission frequency of the light emitting element LD2 adjusted to the emission frequency of the light emitting element LD1.

As described above, while the blood flow is measured based on a change in the amount of received light of the light receiving element 3 caused by an effect of a beat signal or the like, the change in the amount of received light of the light receiving element 3 caused by the effect of the beat signal or the like acts as a noise component in the measurement of the oxygen saturation in the blood.

Therefore, hereinafter, a living-body information measurement device for measuring plural living-body information with high accuracy even when a frequency-varying component such as a beat signal is contained in the received signal from the light receiving element 3 will be described.

FIG. 8 is a view illustrating the configuration of a living-body information measurement device 10 according to an exemplary embodiment.

As illustrated in FIG. 8, the living-body information measurement device 10 includes a control unit 12, a drive circuit 14, an amplification circuit 16, an analog/digital (A/D) conversion circuit 18, a measuring unit 20, a signal separation circuit 22, a low pass filter (LPF) 24, a light emitting element LD1, a light emitting element LD2, and the light receiving element 3.

The control unit 12 outputs a control signal, which controls a light-emission period and emission interval of each of the light emitting elements LD1 and LD2, to the drive circuit 14 including a power supply circuit for supplying drive power to the light emitting elements LD1 and LD2.

Upon receiving the control signal from the control unit 12, according to the light-emission period and emission interval instructed by the control signal, the drive circuit 14 supplies the drive power to the light emitting elements LD1 and LD2 so as to drive the light emitting elements LD1 and LD2.

FIG. 9 illustrates one example of arrangement of the light emitting elements LD1 and LD2 and the light receiving element 3 in the living-body information measurement device 10. As illustrated in FIG. 9, the light emitting elements LD1 and LD2 and the light receiving element 3 are arranged side by side on the living body 8. In this example, the light receiving element 3 receives light of the light emitting elements LD1 and LD2 which is reflected at the living body 8.

However, the arrangement of the light emitting elements LD1 and LD2 and the light receiving element 3 is not limited to the arrangement example of FIG. 9. For example, as illustrated in FIG. 10, the light emitting elements LD1 and LD2 may be arranged to face the light receiving element 3 with the living body 8 sandwiched therebetween. In this example, the light receiving element 3 receives light of the light emitting elements LD1 and LD2 which transmits through the living body 8.

Although, in these examples, the light emitting elements LD1 and LD2 are both vertical cavity surface-emission lasers, the light emitting elements LD1 and LD2 are not limited thereto but may be edge-emission lasers.

When the blood flow is to be measured by the measuring unit 20, since this measurement is made based on a spectrum distribution of the amount of received light according to a beat signal, a laser device which may generate a beat signal more easily than different light may be preferably used for the light emitting element LD1.

However, even if the light emitted from the light emitting element LD2 is not a laser beam, since the amount of light absorbed variation ΔA_(Red) of the light emitting element LD2 may be calculated, a light emitting diode (LED) or an organic light emitting diode (OLED) may be used for the light emitting element LD2.

The amplification circuit 16 converts a current corresponding to the intensity of light received in the light receiving element 3 into a voltage and then amplifies the voltage up to a voltage level specified as an input voltage range of the A/D conversion circuit 18. In other words, the amplification circuit 16 amplifies the received signal output from the light receiving element 3. Although it is here illustrated, as one example, that the light receiving element 3 outputs the current corresponding to the intensity of the received light as a received light signal, the light receiving element 3 may output a voltage corresponding to the intensity of the received light as a received light signal.

The A/D conversion circuit 18 outputs received light signal, which is obtained by digitizing the amount of light received in the light receiving element 3 representing corresponding received light signal, with the received light signal amplified at the amplification circuit 16 as an input.

Upon receiving the digitized received light signal of the light receiving element 3 from the A/D conversion circuit 18, the signal separation circuit 22 separates the received light signal into a received light signal 46 by the light emitting element LD1 which is expressed by a data string of the amount of received light of the light emitting element LD1 and a received light signal 46 by the light emitting element LD2 which is expressed by a data string of the amount of received light of the light emitting element LD2. In addition, whether the amount of the received light signal received from the A/D conversion circuit 18 is contained in the received light signal 46 or the received light signal 48 may be determined, for example, from the light-emission period and emission interval of each of the light emitting elements LD1 and LD2.

The signal separation circuit 22 outputs the received light signal 46 by the light emitting element LD1 to the measuring unit 20 and outputs the received light signal 46 by the light emitting element LD1 and the received light signal 48 by the light emitting element LD2 to the LPF 24.

The LPF 24 attenuates the frequency component that is higher than a predetermined cut-off frequency fc with respect to the frequency component included in a change in the received light signal 46 by the light emitting element LD1 and the frequency component included in a change in the received light signal 48 by the light emitting element LD2, and then outputs the attenuated frequency component to the measuring unit 20. Here, the frequency component of higher than the cut-off frequency fc is referred to as a “high frequency component.” The cut-off frequency fc may be set to about 10 Hz or lower, as will be described later.

The measuring unit 20 includes a blood flow measuring unit 20A and an oxygen saturation measuring unit 20B. The received light signal 46 by the light emitting element LD1, which is output from the signal separation circuit 22, is input to the blood flow measuring unit 20A. In addition, the received light signal 46B by the light emitting element LD1 and the received light signal 48B by the light emitting element LD2, which are output from the LPF 24, are input to the oxygen saturation measuring unit 20B.

Upon receiving the received light signal 46 by the light emitting element LD1, the blood flow measuring unit 20A calculates a spectrum distribution for each frequency ω by subjecting the received light signal 46 to an FFT, and measures the blood flow by integrating the product of the frequency ω and the spectrum intensity at the frequency ω with respect to the frequency ω.

In addition, upon receiving the received light signal 46B by the light emitting element LD1 and the received light signal 48B by the light emitting element LD2, whose high frequency components are removed by the LPF 24, the oxygen saturation measuring unit 20B measures an oxygen saturation by calculating a variation amount of amount of light absorbed of the light emitting element. LD1 ΔA_(IR) and a variation amount of amount of light absorbed of the light emitting element LD2 ΔA_(Red) according to the equation (1) and calculating a ratio of amount of light absorbed variation amount ΔA_(Red) to amount of light absorbed variation amount ΔA_(IR) according to the equation (2). In addition, the oxygen saturation measuring unit 20B may measure an oxygen saturation in the blood by calculating a known equation which is obtained by modifying the equations (1) and (2).

As described above, since the frequency difference Δω₀ of the beat signal included in the received light signal 46 of the light emitting element LD1 falls within a frequency range having the upper limit of about several tens kHz, a noise component due to the beat signal is removed by the LPF 24. In addition, a noise component due to the beat signal included in the received light signal 48 of the light emitting element LD2 is removed by the LPF 24. Therefore, the oxygen saturation measuring unit 20B may use the received light signal 46B and the received light signal 48B, whose noise components due to the beat signal are removed, to measure the oxygen saturation in the blood.

In addition, when an LED or an OLED is used as the light emitting element LD2, since the light emitted from the light emitting element LD2 may not be a coherent light, it is hard for the beat signal to be included in the received light signal 48 by the light emitting element LD2. Therefore, in this case, the signal separation circuit 22 may directly output the received light signal 48 to the oxygen saturation measuring unit 20B without outputting the signal to the LPF 24.

In addition, even in the blood flow measuring unit 20A, the frequency component that is higher than the frequency difference Δω₀ of the beat signal may act as a noise component in the measurement of the blood flow. Therefore, unlike the LPF 24, another LPF having a cut-off frequency fc of about several tens kHz may be interposed between the signal separation circuit 22 and the blood flow measuring unit 20A.

In addition, considering a tendency that a frequency component has a lower correlation with the accuracy of measurement of the blood flow as the frequency component become closer to a DC, instead of the above-mentioned another LPF, a band pass filter that passes a frequency component of several Hz to several tens kHz may be interposed between the signal separation circuit 22 and the blood flow measuring unit 20A to thereby remove the DC component of lower than several Hz and the frequency component of higher than several tens kHz from the received light signal 46.

In this case, the measurement accuracy of the blood flow in the living-body information measurement device 10 may be enhanced over a case where the above-mentioned another LPF or band pass filter is not provided.

As one example, when the light emitting element LD1 and the light emitting element LD2 emit light alternately as illustrated in FIG. 11, an example of output waveforms of the received light signal 46 by the light emitting element LD1 in the LPF 24 is illustrated in FIG. 12. In FIG. 11, plural points 96 indicate light receiving points 96 by the light receiving element 3.

The output waveforms of the received light signal 46B illustrated in FIG. 12 are output waveforms obtained respectively when the cut-off frequency fc of the LPF 24 is set to 5 Hz, 10 Hz, 200 Hz and FULL. As used herein, “FULL” of the cut-off frequency fc refers to that the cut-off frequency fc is at infinity, i.e., the received light signal 46 input to the LPF 24 is output as it is.

As illustrated in FIG. 12, as the cut-off frequency fc becomes lower, the noise component is removed from the output waveforms of the received light signal 46, thereby making the output waveforms smoother. In this case, even when the cut-off frequency fc is 200 Hz, it may be seen that a noise component is yet included in the received light signal 46. Therefore, it is preferable that the cut-off frequency fc falls within a range of from about 5 Hz to about 10 Hz.

As described above, with the living-body information measurement device 10 according to the exemplary embodiment, a received light signal of the light emitting element 1 after passing through the TPF 24 is used to measure the oxygen saturation in the blood. Therefore, in the measurement of the living-body information, even when a beat signal caused by an interference of coherent light emitted from the light emitting element 1 acts as a noise component, since the noise component is removed by the LPF 24, it is possible to accurately measure the living-body information of interest.

In addition, the living-body information measurement device 10 may be used for measurement of the blood velocity, in addition to the blood flow, as described above. In addition, as illustrated in FIG. 7, since the amount of light received in the light receiving element 3 varies depending on the pulse of the arteries 4, it is possible to measure a pulse rate from the variation of the amount of light received in the light receiving element 3. In addition, it is possible to measure a photoelectric pulse wave by differentiating a waveform obtained by measuring a change in pulse rate in a chronological order twice. The photoelectric pulse wave is used for estimation of blood vessel age, diagnosis of arteriosclerosis, or the like.

In addition, the living-body information measurement device 10 may be used for measurement of other living body information without being limited to the above-mentioned living-body information.

In addition, the processes of the living-body information measurement device 10 illustrated in FIG. 8 may be implemented with software, hardware, or a combination thereof.

The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalent. 

What is claimed is:
 1. A living-body information measurement device comprising: a first light emitting element and a second light emitting element each that emits different light in wavelength; a light receiving element that receives the light emitted from the first light emitting element and the second light emitting element and outputs received light signal corresponding to an amount of the received light; a separation unit that separates the received light signal into a first received light signal corresponding to the amount of received light emitted from the first light emitting element and a second received light signal corresponding to the amount of received light emitted from the second light emitting element; a filter that removes noise components of the first received light signal and the second received light signal; and a measuring unit that measures a plurality of living-body information using the first received light signal before the noise components are removed by the filter, the first received light signal whose noise components are removed by the filter, and the second received light signal whose noise components are removed by the filter.
 2. The living-body information measurement device according to claim 1, wherein the measuring unit measures the plurality of living-body information using a frequency spectrum of the first received light signal before the noise components are removed by the filter, and a ratio of a change in the second received light signal whose noise components are removed by the filter to a change in the first received light signal whose noise components are removed by the filter.
 3. The living-body information measurement device according to claim 1, wherein the measuring unit measures living-body information including a blood flow or a blood velocity and an oxygen saturation in blood, as the plurality of living-body information, and wherein the oxygen saturation in blood is measured using a ratio of a change in the second received light signal whose noise components are removed by the filter to a change in the first received light signal whose noise components are removed by the filter.
 4. The living-body information measurement device according to claim 1, wherein the measuring unit measures living-body information including a blood flow or a blood velocity and an oxygen saturation in blood, as the plurality of living-body information, and wherein the blood flow or the blood velocity is measured using a frequency spectrum of the first received light signal.
 5. The living-body information measurement device according to claim 1, wherein the filter is a band pass filter.
 6. The living-body information measurement device according to claim 1, wherein the filter is a low pass filter. 